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Calculating a pre-ferment

March 18, 2010 - 1:31pm

Barbara Krauss

Calculating a pre-ferment

I have a question about baker's math and pre-fermented flour. When a recipe calls for "25% fermented flour," what does this mean in terms of baker's math? In other words, how do you compute a formula using that information?

I think I know, but I'm not really sure. I would assume that if you're starting with, say 1000g of flour, then 250 grams of that is fermented using the water from the total water content of the recipe, according to what percentage of hydration you want the pre-ferment to be (125g for a stiff starter, etc.) Is this correct?

I would assume it means that 25% of the total flour is in the preferment so...if the total flour is 1000 grams you would want 25% to be prefermented or 250 grams. Assuming that the preferment was 100%, that would be 500 grams of preferment (250 grams of flour and 250 grams of water). If you wanted the final hydration to be 70% then you would want a total of 700 grams of water (70% of 1000). Since the preferment has 250, you would need to add another 450 grams of water and 750 grams of flour (1000 - 250 in the preferment).

And as messy as that may seem to those who are math averse, it really isn't all that hard!

As a check, this equates to adding about 2.5 times the weight of the preferment when making the final dough Adding 1200 grams (water and flour) to 500 . That is somewhat low expansion for sourdough but...not outrageous. IF the recipe were done the way you suggest the additon would be more like 5 to 6 times the weight of the preferment and that would be challenging for most sourdough yeast to raise gracefully in my experience. In my experience it would tend to need some commercial yeast to give it a boost.

I've found where the term is used interchangably with sourdough (not a sponge or barm) in a few Northern European countries. However, something very interesting...in African countries where other starchy vegetables and grains are used to make flour (such as tubers or amaranth) there is a process of using dried fermented flour in baking (study I read says "oven-dried and sun-dried").

So, depending upon where your recipe orginated, it could mean two completely different things! Sorta like we call Sweet Potatoes "yams" in the Southern US, but they aren't really yams.

Thanks for the analysis, it helps. I have seen the term a number of times in recipes here and have always been a little cloudy as to what it means. Having just recently acquainted myself with the basics of baker's math (finally!) I've struggled with this component. As you and Mary both point out, it might mean different things to different people.

I saw the phrase recently on Joe Va's blog (which he duplicated on this forum) and he makes no reference to any extra yeast in the recipe:

"The bread I baked was based on Shiao-Ping suggestions with the obvious adjustment you have to do every time you bake, with different ingredients and conditions: 85% bread flour, 10% whole wheat, 5% rye. 65% overall hydration. 25% prefermented flour (100% hydration white levain). Short mixing with S&F, 12h retarded at 5°C. I also used the "double flour addition" technique of SteveB."

Maybe if Joe or Shiao-Ping see this, they would be willing to elaborate.

Barbara- I'm going to offer a different interpretation to Jay's, though I agree with him that the phrase is odd. Typically recipes call for a percentage of preferment, and additionally, specify what that is: biga, pate fermentee, poolish, sourdough.

So, if by "fermented flour" the recipe intends preferment then the weight of the preferment is 25% of the total flour weight - so if the TWF is 1000g, then the preferment would be 250g. (Incidentally, that is right in the range of usual preferment percentages).

In that case, the amount of flour would depend on the preferment and its typical hydration. Poolish, for example, is typically around 100% hydration, so this would translate to 125g flour, plus 125g water and a speck of yeast. For a biga, which is a firmer preferment, this might mean 155g of flour to 95g water to yield a 60% hydration.

That makes sense, because the recipe I made reference to above (Joe Va's) did specify 100% hydration. So this would result in 250 grams of a pre-ferment using half that amount of flour and an equal amount of water, and in that way would not throw off the original proportions.

I posed this question to Joe on his blog and as it turns out he did address this very subject in depth with his opening post to this forum. I think it must have flown right over my head at the time but he explains the process here:

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